I cannot see the color bands easily in the photo so the next thing to check is all resistor values by color band since you cannot test them in circuit. You can use the card in the guides section or you can use an online calculator.
The problem you are describing sounds to me like a possible resistance issue.
There are not that many so it should not take too long but do not rely on packaging, labels etc and do them all by color band with the build guide photo in hand and check them off one at a time.
One other thought is the Output Jack looks very close to shorting on the back of the pot where there is no tape especially when inserting a cable which would expand it. Just an after thought.